Valve and fueling strategy for operating a controlled auto-ignition four-stroke internal combustion engine

ABSTRACT

Low load operating point for a controlled auto-ignition four-stroke internal combustion engine is reduced without compromising combustion stability through a valve control operative to establish sub-atmospheric pressure conditions within the combustion chamber into which fuel and exhaust gases are introduced. A split-injection control operative to introduce a first fuel fraction into the combustion chamber during an intake stroke and a second fuel fraction into the combustion chamber during a compression event, in combination with the valve control, provides further reductions in low load operating points without compromising combustion stability.

TECHNICAL FIELD

The present invention is related to operating a four-stroke internalcombustion engine.

BACKGROUND OF THE INVENTION

The automotive industry is continually researching new ways of improvingthe combustion process of the internal combustion engine in an effort toimprove fuel economy and meet or exceed emission regulatory targets, andto meet or exceed consumer expectations regarding emissions, fueleconomy and product differentiation.

Most modern conventional internal combustion engines attempt to operatearound stoichiometric conditions. That is providing an optimal air/fuelratio of substantially 14.6 to 1 that results in substantially completeconsumption of the fuel and oxygen delivered to the engine. Suchoperation allows for exhaust gas aftertreatment by 3-way catalysts whichclean up any unconsumed fuel and combustion byproducts such as NOx andCO. Most modern engines are fuel injected having either throttle bodyinjection (TBI) or multi-port fuel injection (MPFI) wherein each of aplurality of injectors is located proximate an intake port at eachcylinder of a multi-cylinder engine. Better air/fuel ratio control isachieved with a MPFI arrangement; however, conditions such as wallwetting and intake runner dynamics limit the precision with which suchcontrol is achieved. Fuel delivery precision can be improved by directin-cylinder injection (DI). So called linear oxygen sensors provide ahigher degree of control capability and when coupled with DI suggest anattractive system with improved cylinder-to-cylinder air/fuel ratiocontrol capability. However, in-cylinder combustion dynamics then becomemore important and combustion quality plays an increasingly importantrole in controlling emissions. As such, engine manufacturers haveconcentrated on such things as injector spray patterns, intake swirl,and piston geometry to affect improved in-cylinder air/fuel mixing andhomogeneity.

While stoichiometric gasoline four-stroke engine and 3-way catalystsystems have the potential to meet ultra-low emission targets,efficiency of such systems lags behind so-called lean-burn systems.Lean-burn systems also show promise in meeting emission targets for NOxthrough combustion controls, including high exhaust gas dilution andemerging NOx aftertreatment technologies. However, lean-burn systemsstill face other hurdles, for example, combustion quality and combustionstability particularly at low load operating points and high exhaust gasdilution.

Lean-burn engines, at a most basic level, include all internalcombustion engines operated with air in excess of that required for thecombustion of the fuel charge provided. A variety of fueling andignition methodologies differentiate lean-burn topologies. Spark ignitedsystems (SI) initiate combustion by providing an electrical discharge inthe combustion chamber. Compression ignition systems (CI) initiatecombustion by combustion chamber conditions including combinations ofair/fuel ratio, temperature and pressure among others. Fueling methodsmay include TBI, MPFI and DI. Homogeneous charge systems arecharacterized by very consistent and well vaporized fuel distributionwithin the air/fuel mixture as may be achieved by MPFI or directinjection early in the intake cycle. Stratified charge systems arecharacterized by less well vaporized and distributed fuel within theair/fuel mixture and are typically associated with direct injection offuel late in the compression cycle.

Known gasoline DI engines may selectively be operated under homogeneousspark ignition or stratified spark ignition modes. A homogeneous sparkignited mode is generally selected for higher load conditions while astratified spark ignition mode is generally selected for lower loadconditions.

Certain DI compression ignition engines utilize a substantiallyhomogeneous mixture of preheated air and fuel and establish pressure andtemperature conditions during engine compression cycles that causeignition without the necessity for additional spark energy. This processis sometimes called controlled auto-ignition. Controlled auto-ignitionis a predictable process and thus differs from undesirable pre-ignitionevents sometimes associated with spark-ignition engines. Controlledauto-ignition also differs from well-known compression ignition indiesel engines wherein fuel ignites substantially immediately uponinjection into a highly pre-compressed, high temperature charge of air,whereas in the controlled auto-ignition process the preheated air andfuel are mixed together prior to combustion during intake events andgenerally at compression profiles consistent with conventional sparkignited four-stroke engine systems.

Four-stroke internal combustion engines have been proposed which providefor auto-ignition by controlling the motion of the intake and exhaustvalves associated with a combustion chamber to ensure that a air/fuelcharge is mixed with combusted gases to generate conditions suitable forauto-ignition without the necessity for externally pre-heating intakeair or cylinder charge or for high compression profiles. In this regard,certain engine have been proposed having a cam-actuated exhaust valvethat is closed significantly later in the four-stroke cycle than isconventional in a spark-ignited four-stroke engine to allow forsubstantial overlap of the open exhaust valve with an open intake valvewhereby previously expelled combusted gases are drawn back into thecombustion chamber early during the intake cycle. Certain other engineshave been proposed that have an exhaust valve that is closedsignificantly earlier in the exhaust cycle thereby trapping combustedgases for subsequent mixing with fuel and air during the intake cycle.In both such engines the exhaust valve is opened only once in eachfour-stroke cycle. Certain other engines have been proposed having ahydraulically controlled exhaust valve that is opened twice during eachfour-stroke cycle—once to expel combusted gases from the combustionchamber into the exhaust passage during the exhaust cycle and once todraw back combusted gases from the exhaust passage into combustionchamber late during the intake cycle. All of these proposed engines relyupon either throttle body or port fuel injection. Another proposedengine, however, has hydraulically controlled intake and exhaust valveswherein the exhaust valve is opened twice during each four-stroke cycleand additionally utilizes direct combustion chamber fuel injection forinjecting fuel during either the intake or compression cycle.

However advantageous such lean-burn engine systems appear to be, certainshortfalls with respect to combustion quality and combustion stability,particularly at low load operating points and high exhaust gas dilution,continue to exist. Such shortfalls lead to undesirable compromisesincluding limitations on how much a fuel charge can effectively beleaned out during low load operating points while still maintainingacceptable combustion quality and stability characteristics.

SUMMARY OF THE INVENTION

It is recognized that homogeneous air/fuel charges within a combustionchamber are generally desirable in a variety of internal combustionengines, including engines employing strategies such as TBI, MPFI, DI,SI, CI, controlled auto-ignition, stoichiometric, lean-burn andcombinations and variants thereof. A lean-burn four-stroke internalcombustion engine is generally desirable. Furthermore, such an engineexhibiting high combustion stability at low load operating points isdesirable. Moreover, such an engine capable of extended lean operationinto heretofore unattained low load operating point regions isdesirable.

The present invention provides these and other desirable aspects in amethod of operating a four-stroke internal combustion engine withextended capability at low engine loads while maintaining or improvingcombustion quality, combustion stability and NOx emissions.

In accordance with one aspect the present invention, a low pressureevent is established within the combustion chamber during the intakestroke of the piston. The depth and duration of the low pressure eventdirectly affects the combustion stability and low load limit of theengine. Intake and exhaust valve phasing, or opening and closing timingis used to establish the low pressure event profiles. Exhaust gases fromthe engine are recirculated into the combustion chamber during theintake cycle. A rebreathe event whereby the intake valve is openedduring the exhaust cycle provides internal recirculation by expellingcombusted gases into the intake passage for subsequent recirculation orrebreathe thereof by drawing them back into the combustion chambervis-a-vis the intake valve opening during the intake event.

In accordance with another aspect of the present invention, asplit-injection strategy is employed whereby a first fraction of fuel isinjected early during the intake cycle and the remaining fuel of thetotal fuel charge for the cycle is injected late during the compressioncycle. The total net mean effective pressure, which is a directindicator of engine load and total fuel, required to maintain acceptablecombustion stability is significantly less than the requirements ofsimilar conventionally fueled engines.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a single cylinder,direct-injection, four-stroke internal combustion engine in accordancewith the present invention;

FIG. 2 illustrates various valve lift versus crank angle curvescorresponding to exemplary exhaust and intake valve phasing of thesingle cylinder engine of FIG. 1 in accordance with the presentinvention;

FIG. 3 illustrates a cylinder pressure versus crank angle curvecorresponding to the exemplary exhaust and intake valve phasingillustrated in FIG. 2 in accordance with the present invention; and,

FIG. 4 illustrates exemplary combustion stability versus cylinder netmean effective pressure curves demonstrative of low load limit benefitsin accordance with the phase control aspect and combined phase controland fueling aspects in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference first to FIG. 1, an exemplary single cylinder four-strokeinternal combustion engine system (engine) 10 suited for implementationof the present invention is schematically illustrated. It is to beappreciated that the present invention is equally applicable to amulti-cylinder four-stroke internal combustion engine. The presentexemplary engine 10 is shown configured for direct combustion chamberinjection (direct injection) of fuel vis-a-vis fuel injector 41.Alternative fueling strategies including port fuel injection or throttlebody fuel injection may also be used in the implementation of thepresent invention; however, the preferred approach is direct injection.Similarly, while widely available grades of gasoline and light ethanolblends thereof are preferred fuels, alternative liquid and gaseous fuelssuch as higher ethanol blends (e.g. E80, E85), neat ethanol (E99), neatmethanol (M100), natural gas, hydrogen, biogas, various reformates,syngases etc. may also be used in the implementation of the presentinvention.

With respect to the base engine, a piston 11 is movable in a cylinder 13and defines therein a variable volume combustion chamber 15. Piston 11is connected to crankshaft 35 through connecting rod 33 and reciprocallydrives or is reciprocally driven by crankshaft 35. Engine 10 alsoincludes valve train 16 illustrated with a single intake valve 21 and asingle exhaust valve 23 though multiple intake and exhaust valvevariations are equally applicable for utilization with the presentinvention. Valve train 16 also includes valve actuation means 25 whichmay take any of a variety of forms including, preferably, electricallycontrolled hydraulic or electromechanical actuation. Alternative valveactuation means adaptable for implementation in conjunction with thepresent invention include multi-profile cams, cam phasers and othermechanically variable valve actuation technologies implementedindividually or in combination with another. Intake passage 17 suppliesair into the combustion chamber 15. The flow of the air into thecombustion chamber 15 is controlled by intake valve 21 during intakeevents. Combusted gases are expelled from the combustion chamber 15through exhaust passage 19 with flow controlled by exhaust valve 23during exhaust events.

Engine control is provided by computer based control 27 which may takethe form of conventional hardware configurations and combinationsincluding powertrain controllers, engine controllers and digital signalprocessors in integrated or distributed architectures. In general,control 27 includes at least one microprocessor, ROM, RAM, and variousI/O devices including A/D and D/A converters and power drive circuitry.Control 27 also specifically includes controls for valve actuation means25 and fuel injector 41. Controller 27 includes the monitoring of aplurality of engine related inputs from a plurality of transducedsources including engine coolant temperature, outside air temperature,manifold air temperature, operator torque requests, ambient pressure,manifold pressure in throttled applications, displacement and positionsensors such as for valve train and engine crankshaft quantities, andfurther includes the generation of control commands for a variety ofactuators as well as the performance of general diagnostic functions.While illustrated and described as integral with controller 27, thecontrol and power electronics associated with valve actuation means 25and fuel injector 41 may be incorporated as part of distributed smartactuation scheme wherein certain monitoring and control functionalityrelated to respective subsystems are implemented by programmabledistributed controllers associated with such respective valve and fuelcontrol subsystems.

Having thus described the environment and certain application hardwaresuitable for implementing the method of the present invention, FIGS. 2and 3 are now referenced to describe the method itself. In FIG. 2, valvelifts of the intake and exhaust valves are plotted against a completefour-stroke combustion cycle. A full 720 degrees or two revolutions ofthe crankshaft are plotted against the horizontal axis beginning at 0degrees corresponding to top dead center (TDC) of the piston at thebeginning of the expansion stroke (end of the compression stroke)through to the same top dead center position at the end of thecompression stroke (beginning of the expansion stroke). By conventionand as followed herein, the crankshaft angular positions 0 through 720refer to degrees of crankshaft rotation after top dead centercombustion. The sequentially repeated cycles are delineated across thetop of the figure within double-ended arrows labeled Expansion, Exhaust,Intake and Compression. Each of these cycles correspond to the pistonmotion between respective ones of top dead and bottom dead centerpositions and covers a full 180 degrees of crankshaft rotation orone-quarter of the complete four-stroke cycle. In FIG. 3, cylinderpressures are plotted against contiguous portions of the four-strokecombustion cycle, to wit the exhaust and intake strokes as clearlyevidenced by the similarly labeled double-ended arrows shown across thetop of the figure.

In the present exemplary exposition of the invention, a four-strokesingle cylinder, 0.55 liter, controlled auto-ignition, gasoline fueledinternal combustion engine was utilized in implementing the variousvalve controls and acquisition of the various data embodied herein.Unless specifically discussed otherwise, all such implementations andacquisitions are assumed to be carried out under standard conditions asunderstood by one having ordinary skill in the art.

In accordance with the valve phase control aspects of the presentinvention a low pressure event is established within the combustionchamber, preferably by means of phase control over the opening andclosing of one or more of the intake and exhaust valves. In the presentexample illustrated in FIGS. 2 and 3, it is assumed that an exhaustevent is caused to occur wherein the exhaust valve is opened for atleast a portion of the exhaust stroke from 120 to 270 degrees. Theactual opening and closing angles of the exhaust valve during an exhaustevent will vary in accordance with such factors as engine speed or loadand exhaust runner geometries as well as other desired engine tuningcharacteristics. In the present illustrated example the exhaust valveclosure is assumed to correspond substantially to 370 degrees or 10degrees after exhaust stroke TDC. Preferably, the exhaust valve closureoccurs within approximately 20 degrees before exhaust stroke TDC to 20degrees after exhaust stroke TDC. It is generally believed that maximumexpulsion of exhaust gases from the combustion chamber will aid inminimizing residual cylinder pressure and such condition is generallyconsistent with effectuating deeper and longer duration low pressureevents. Through certain gas dynamics under certain conditions maximumexpulsion occurs when the exhaust valve remains open for some angleafter exhaust stroke TDC. More preferably, then, the exhaust valveclosure occurs within approximately exhaust stroke TDC to 20 degreesafter exhaust stroke TDC.

Consistent with the objective of establishing a low pressure eventwithin the combustion chamber during the intake stroke it may further bedesirable that the exhaust event exhaust valve closure absolute phasebefore exhaust stroke TDC is not greater than the intake valve openingphase after exhaust stroke TDC or that minimal valve overlap exists.Generally a certain degree of asymmetry around exhaust stroke TDC asbetween exhaust valve closure and intake valve opening as described isrequired in order to establish the desired low pressure conditionswithin the combustion chamber. If exhaust event exhaust valve closureoccurs before exhaust stroke TDC, then it may be desirable to allow atleast a similar angle after TDC for the pressure in the combustionchamber to relax before the intake valve begins to open. Preferably, theintake valve opening during an intake event follows the exhaust valveclosing at about 30 to about 90 degrees after exhaust stroke TDC at lowload operating points.

In accordance with another feature of the present invention the intakevalve is opened during at least a portion of the exhaust event to expelcombusted gases into the intake passage 17 for subsequent recirculationor rebreathe thereof by drawing them back into the combustion chambervis-a-vis the intake valve. Preferably, this rebreathe event intakevalve opening occurs subsequent to the opening of the exhaust valve andmore preferably occurs about 20 degrees before to about 40 degrees afterthe expansion stroke bottom dead center (BDC). Additionally, the intakevalve closing associated with this rebreathe event preferably occurs insubstantial unison with the closure of the exhaust valve or withinapproximately 20 degrees before to about 20 degrees after exhaust strokeTDC. More preferably, then, the intake valve closing associated withthis rebreathe event occurs within approximately exhaust stroke TDC toabout 20 degrees after exhaust stroke TDC.

The rebreathe event intake valve opening is also preferablycharacterized by a relatively low valve lift. More preferably such valvelift is no greater than about 50% of maximum valve lift.

The general and preferred intake and exhaust valve phasings heretoforedescribed are substantially set forth in the exemplary curvesillustrated in FIG. 2. Curve 50 represents an exhaust event exhaustvalve profile wherein valve closure occurs at substantially 10 degreesafter exhaust stroke TDC. For purposes of exposition it is assumed thatthe exhaust event is substantially static with respect to exhaust eventexhaust valve closure phasing although, as described previously, it iscontemplated that in fact phase shifting of the exhaust valve closure iswithin the scope of the invention in attaining various outcomes andobjectives thereof. Intake profile 51 corresponds to an exemplary intakevalve opening at substantially 70 degrees after exhaust stroke TDC andclosing at substantially 40 degrees after intake stroke BDC. Rebreatheprofiles 52 correlates to the exhaust valve opening profile 50 andcorresponds to a rebreathe event intake valve opening initiated at about20 degrees before the expansion stroke BDC. Rebreathe profile 52 andexhaust valve profile 50 also illustrate substantial convergence ofrespective closures at about 10 degrees after the exhaust stroke TDC.

If a continuum of such intake profiles were plotted in the figure withintake valve opening limits at less retarded and more retarded phaseangles, for example from 380 degrees to 450 degrees, the result would bevarying vacuum levels and durations thereof within the combustionchamber. Of course, in addition to the various low pressure profileswithin the combustion chamber which can be achieved with simply phaseshifting valve openings as described, additional pressure profiles maybe achieved through more complex and independent variations of theexhaust and intake profiles including by way of lift variation inaddition to timing. It should be noted also that significant variationsin gas constituent mixtures and temperature can also be effected by wayof the complex variations of the exhaust, intake and rebreathe profilesthat are possible. The operation of the engine as exhibited by theexemplary figures herein is, as indicated earlier, as a controlledauto-ignition engine. Additional details respecting varying vacuumlevels is set forth in additional detail in commonly assigned andco-pending U.S. patent application Ser. No. 10/611,845 (Attorney DocketNumber GP-303270), the contents of which are incorporated herein byreference.

The valve phase controls to establish a low pressure event within thecombustion chamber are carried out to establish pressure leveldepressions and durations thereof within the combustion chamber that arenot found in conventional known four-stroke operation. With referencenow to FIG. 3, a pressure profile resulting from the exemplary valveprofiles described with respect to FIG. 2 is illustrated. Therein, acurve is generally designated by the numeral 61 and is illustrated withrespect to 360 degrees of crankshaft rotation, to wit through theexhaust and intake strokes of the complete four-stroke process only asdelineated across the top of the figure within double-ended arrowslabeled Exhaust and Intake. Cylinder pressure is illustrated on arelative linear scale along the vertical axis with ambient pressurebeing specifically labeled and assumed to be substantially one standardatmosphere or about 101 kPa. Region 63 generally designates the area ofresultant low pressure events or sub-atmospheric pressure conditionsestablished in accordance with the present invention. A moderately deepand lasting duration low pressure event is sub-atmospheric fromsubstantially just after 390 degrees to substantially just prior to 500degrees and reaches substantially 60 kPa below ambient orsub-atmospheric or alternatively stated about 60% below ambient oratmospheric or about 40% of ambient or atmospheric. The specific curveillustrated in FIG. 3 is, of course, exemplary with other such curvesand profiles being able to be established by virtue of more complex andindependent variations of the exhaust, intake and rebreathe profilesincluding by way of lift variation in addition to timing. For example,relative to the specific curve illustrated in FIG. 3, further retardingthe intake valve opening during intake events would effectuate deeperlow pressure events whereas further advancing the intake valve openingduring intake events would effectuate shallower low pressure events. Anexemplary, relatively shallow and limited duration low pressure event issub-atmospheric through substantially just after 435 degrees or 75degrees past exhaust stroke TDC, and reaches substantially 42 kPa belowambient or sub-atmospheric or alternatively stated about 42% belowambient or atmospheric or about 58% of ambient or atmospheric. Anexemplary, relatively deep and lasting duration low pressure event issub-atmospheric through substantially just prior to 480 degrees or 120degrees past exhaust stroke TDC, and reaches substantially 75 kPa belowambient or sub-atmospheric or alternatively stated about 75% belowambient or atmospheric or about 25% of ambient or atmospheric.

The fueling methodology for an engine operated as described may beselected from any variety of methods. Liquid and gaseous injections arecandidates for DI. Additionally, it is contemplated that air assistedand other types of delivery may be employed. Also, the type of ignitionsystem employable is variable and includes such non-limiting examples asSI, CI, and controlled auto-ignition.

The impact of the valve phase control aspects of the current inventionon the low load limit of the exemplary controlled auto-ignition engineoperation is shown in FIG. 4. Without using the valve phasing of thecurrent invention, the low load limit of the exemplary—and mosttypical—four-stroke direct-injection auto-ignition gasoline engine isaround 240 kPa Net Mean Effective Pressure (NMEP) with a generallyaccepted 5% Coefficient of Variation of Indicated Mean EffectivePressure (COV of IMEP) as an indicator. With the present invention, itis possible to reduce fueling to substantially 165 kPa NMEP for eitherthrottle body or port fuel injection. FIG. 4 is demonstrative of thesebenefits. In that figure, point 83 represents the low load limit ofsubstantially 240 kPa in terms of NMEP with 5% COV of IMEP as theindicator. Points to the left in the figure (i.e. lower NMEP) correspondto lower loads attained using valve phase control aspects of the presentinvention. The plot of line 81 clearly shows significantly lower NMEPbefore the acceptable 5% or less COV of IMEP is reached, effectivelymoving the low load limit point to about 165 kPa NMEP.

In accordance with the fueling control aspects of the present invention,a split-injection of the total fuel charge is caused to occur. That is,the total fuel requirement for the cycle is divided into two injectionevents. One of the injection events is carried out early in the intakecycle while the other injection event is carried out late in thecompression cycle. Generally, the intake cycle fueling event injectsabout 10 to about 50 percent of the total fuel requirement for thecycle. Generally, the cylinder charge established by this first fractionof fuel is insufficient for auto-ignition within the combustion chamber.The remainder of the fuel requirement for the cycle is injected duringthe compression fueling event. This second fraction of fuel enriches thecylinder charge during a compression stroke of the piston sufficient tocause auto-ignition. The total fueling requirement (i.e. the combinedfirst and second fuel fractions) is significantly less than the fuelingrequirement of a similar conventionally operated internal combustionengine as determined against such common metrics as combustion stabilityas will be demonstrated later with respect to FIG. 4. This is true interms of comparative absolute mass of fuel for similar base engines orin terms of relative metrics such as net mean effective pressures.

FIG. 2 is demonstrative of exemplary split-fueling in accordance withcertain preferences regarding injection timing. The region delimited bythe solid bars labeled 55 and 57 correspond to preferred angular regionswithin the intake and compression cycles for delivery of the intakecycle fueling event and compression cycle fueling event, respectively.Preferably, the first fraction of fuel is injected about 0 to about 90degrees after exhaust stroke TDC and the second fraction of fuel isinjected about 20 to about 60 degrees before compression stroke TDC.Other regions for injection may be utilized but may not yield assubstantial an advantage as the preferred regions.

With reference again to FIG. 4, the plot of line 85 also clearly showssignificantly lower NMEP required to maintain an acceptable 5% or lessCOV of IMEP effectively moving the low load limit point to about 25 kPaNMEP when the split-injection strategy of the present invention iscombined with the establishment of the low-pressure conditions withinthe combustion chamber.

The present invention has been described with respect to certainpreferred embodiments and variations herein. Other alternativeembodiments, variations ad implementations may be implemented andpracticed without departing from the scope of the invention which is tobe limited only by the claims as follow:

1. Method of operating a four-stroke internal combustion engineincluding a variable volume combustion chamber defined by a pistonreciprocating within a cylinder between top-dead center and bottom-deadcenter points and an intake valve and an exhaust valve controlled duringrepetitive, sequential exhaust, intake, compression and expansionstrokes of said piston comprising: providing an exhaust event duringwhich the exhaust valve is open for expelling combusted gases from thecombustion chamber; providing a rebreathe event wherein the intake valveis open during at least a portion of at least one of the expansion andexhaust strokes for expelling a portion of combusted gases from thecombustion chamber to an intake passage; subsequent to the exhaustevent, providing a period of simultaneous closure of the exhaust andintake valves during at least a portion of the intake stroke of thepiston effective to establish a sub-atmospheric pressure conditionwithin the combustion chamber; and, providing an intake event duringwhich the intake valve is open for ingesting fresh air and combustedgases from the intake passage into the combustion chamber.
 2. Method ofoperating a four-stroke internal combustion engine as claimed in claim 1wherein said sub-atmospheric pressure condition within the combustionchamber reaches at least about 42 kPa sub-atmospheric.
 3. Method ofoperating a four-stroke internal combustion engine as claimed in claim 1wherein said sub-atmospheric pressure condition within the combustionchamber terminates not earlier than about 75 degrees past exhaust stroketop dead center.
 4. Method of operating a four-stroke internalcombustion engine as claimed in claim 1 wherein said sub-atmosphericpressure condition within the combustion chamber reaches at least about42 kPa sub-atmospheric and terminates not earlier than about 75 degreespast exhaust stroke top dead center.
 5. Method of operating afour-stroke internal combustion engine as claimed in claim 1 whereinsaid rebreathe event is initiated subsequent to initiation of saidexhaust event.
 6. Method of operating a four-stroke internal combustionengine as claimed in claim 5 wherein said rebreathe event is terminatedsubstantially contemporaneously with termination of said exhaust event.7. Method of operating a four-stroke internal combustion engine asclaimed in claim 1 wherein said rebreathe event is characterized by liftof said intake valve no greater than about 50% of maximum valve lift. 8.Method of operating a four-stroke internal combustion engine as claimedin claim 5 wherein said rebreathe event is initiated about 20 degreesbefore to about 40 degrees after the expansion stroke bottom deadcenter.
 9. Method of operating a four-stroke internal combustion engineas claimed in claim 5 wherein said exhaust event is initiated about 40to about 50 degrees before expansion stroke bottom dead center. 10.Method of operating a four-stroke internal combustion engine as claimedin claim 5 wherein said rebreathe event is initiated about 20 to about90 degrees subsequent to initiation of said exhaust event.
 11. Method ofoperating a four-stroke internal combustion engine as claimed in claim 8wherein said intake event terminates substantially contemporaneouslywith termination of said exhaust event.
 12. Method of operating afour-stroke internal combustion engine as claimed in claim 9 whereinsaid intake event terminates substantially contemporaneously withtermination of said exhaust event.
 13. Method of operating a four-strokeinternal combustion engine as claimed in claim 10 wherein said intakeevent terminates substantially contemporaneously with termination ofsaid exhaust event.
 14. Method of operating a four-stroke internalcombustion engine as claimed in claim 1 wherein said exhaust event isinitiated about 40 to about 50 degrees before expansion stroke bottomdead center, and said rebreathe event is initiated about 20 degreesbefore to about 40 degrees after the expansion stroke bottom deadcenter.
 15. Method of operating a four-stroke internal combustion engineas claimed in claim 1 wherein said exhaust event is initiated about 40to about 50 degrees before expansion stroke bottom dead center, and saidrebreathe event is initiated about 20 degrees to about 90 degreessubsequent to initiation of said exhaust event.
 16. Method of operatinga four-stroke internal combustion engine as claimed in claim 14 whereinsaid rebreathe event is terminated substantially contemporaneously withtermination of said exhaust event.
 17. Method of operating a four-strokeinternal combustion engine as claimed in claim 15 wherein said rebreatheevent is terminated substantially contemporaneously with termination ofsaid exhaust event.
 18. Method of operating a four-stroke, internalcombustion engine as claimed in claim 1 further comprising: providing tothe combustion chamber during the intake stroke a first fraction of fuelof a total combustion cycle fuel requirement; and providing to thecombustion chamber during the compression stroke a second fraction offuel of about the difference between the total combustion cycle fuelrequirement and said first fraction of fuel.
 19. Method of operating afour-stroke, internal combustion engine as claimed in claim 18 whereinthe first fraction of fuel comprises about 10 to about 50 percent of thetotal combustion cycle fuel requirement.
 20. Method of operating adirect-injection, four-stroke, internal combustion engine as claimed inclaim 18 wherein said first fraction of fuel is injected about 0 toabout 90 degrees after exhaust stroke top dead center.
 21. Method ofoperating a direct-injection, four-stroke, internal combustion engine asclaimed in claim 18 wherein the first fraction of fuel comprises about10 to about 50 percent of the total combustion cycle fuel requirementand further wherein said first fraction of fuel is injected about 0 toabout 90 degrees after exhaust stroke top dead center.
 22. Method ofoperating a direct-injection, four-stroke, internal combustion engine asclaimed in claim 18 wherein the second fraction of fuel is injectedabout 20 to about 60 degrees before the compression stroke top deadcenter.
 23. Method of operating a direct-injection, four-stroke,internal combustion engine as claimed in claim 19 wherein the secondfraction of fuel is injected about 20 to about 60 degrees before thecompression stroke top dead center.
 24. Method of operating adirect-injection, four-stroke, internal combustion engine as claimed inclaim 20 wherein the second fraction of fuel is injected about 20 toabout 60 degrees before the compression stroke top dead center. 25.Method of operating a direct-injection, four-stroke, internal combustionengine as claimed in claim 21 wherein the second fraction of fuel isinjected about 20 to about 60 degrees before the compression stroke topdead center.
 26. Method of operating a four-stroke internal combustionengine including a variable volume combustion chamber defined by apiston reciprocating within a cylinder between top-dead center andbottom-dead center points and an intake valve and an exhaust valvecontrolled during repetitive, sequential exhaust, intake, compressionand expansion strokes of said piston comprising: establishing a lowpressure event within the combustion chamber during the intake stroke ofthe piston; and, establishing a combustion chamber rebreathe eventduring the exhaust stroke of the piston by controlling a rebreathe eventintake valve opening and closing.
 27. Method of operating a four-strokeinternal combustion engine as claimed in claim 26 wherein said lowpressure event is established by controlling phasing of exhaust eventexhaust valve closure and intake event intake valve opening.
 28. Methodof operating a four-stroke internal combustion engine as claimed inclaim 27 wherein the exhaust valve closure absolute phase relative toexhaust stroke top dead center is not greater than the intake eventintake valve opening phase after exhaust stroke top dead center. 29.Method of operating a four-stroke internal combustion engine as claimedin claim 28 wherein the exhaust valve closure occurs before exhauststroke top dead center.
 30. Method of operating a four-stroke internalcombustion engine as claimed in claim 28 wherein the exhaust valveclosure occurs after exhaust stroke top dead center.
 31. Method ofoperating a four-stroke internal combustion engine as claimed in claim30 wherein the intake event intake valve opening occurs about 0 to about90 after the exhaust valve closure.
 32. Method of operating afour-stroke internal combustion engine as claimed in claim 31 whereinthe exhaust valve closure occurs about exhaust stroke top dead center toabout 20 degrees-after exhaust stroke top dead center.
 33. Method ofoperating a four-stroke internal combustion engine as claimed in claim27 wherein the rebreathe event intake valve opening occurs about 20 toabout 90 degrees after the exhaust valve opening.
 34. Method ofoperating a four-stroke internal combustion engine as claimed in claim27 wherein the intake event intake valve opening occurs about 30 degreesto about 90 degrees after exhaust stroke top dead center.
 35. Method ofoperating a four-stroke internal combustion engine as claimed in claim27 wherein exhaust valve closure occurs about exhaust stroke top deadcenter to about 20 degrees after exhaust stroke top dead center and theintake event intake valve opening occurs about 30 degrees to about 90degrees after exhaust stroke top dead center.
 36. Method of operating afour-stroke internal combustion engine as claimed in claim 27 whereinthe rebreathe event intake valve opening occurs about 20 degrees beforeto about 20 degrees after expansion stroke bottom dead center. 37.Method of operating a four-stroke internal combustion engine as claimedin claim 27 wherein said exhaust event is initiated about 40 to about 50degrees before expansion stroke bottom dead center and is terminatedabout exhaust stroke top dead center to about 20 degrees after exhauststroke top dead center, and said rebreathe event is initiated about 20degrees before to about 40 degrees after the expansion stroke bottomdead center.
 38. Method of operating a four-stroke internal combustionengine as claimed in claim 27 wherein said exhaust event is initiatedabout 40 to about 50 degrees before expansion stroke bottom dead centerand is terminated about exhaust stroke top dead center to about 20degrees after exhaust stroke top dead center, and said rebreathe eventis initiated about 20 to about 90 degrees after the exhaust valveopening.
 39. Method of operating a four-stroke, internal combustionengine as claimed in claim 26 further comprising: providing to thecombustion chamber during the intake stroke a first fraction of fuel ofa total combustion cycle fuel requirement; and providing to thecombustion chamber during the compression stroke a second fraction offuel of about the difference between the total combustion cycle fuelrequirement and said first fraction of fuel.
 40. Method of operating afour-stroke, internal combustion engine as claimed in claim 39 whereinthe first fraction of fuel comprises about 10 to about 50 percent of thetotal combustion cycle fuel requirement.
 41. Method of operating adirect-injection, four-stroke, internal combustion engine as claimed inclaim 39 wherein said first fraction of fuel is injected about 0 toabout 90 degrees after exhaust stroke top dead center.
 42. Method ofoperating a direct-injection, four-stroke, internal combustion engine asclaimed in claim 39 wherein the first fraction of fuel comprises about10 to about 50 percent of the total combustion cycle fuel requirementand further wherein said first fraction of fuel is injected about 0 toabout 90 degrees after exhaust stroke top dead center.
 43. Method ofoperating a direct-injection, four-stroke, internal combustion engine asclaimed in claim 39 wherein the second fraction of fuel is injectedabout 20 to about 60 degrees before the compression stroke top deadcenter.
 44. Method of operating a direct-injection, four-stroke,internal combustion engine as claimed in claim 40 wherein the secondfraction of fuel is injected about 20 to about 60 degrees before thecompression stroke top dead center.
 45. Method of operating adirect-injection, four-stroke, internal combustion engine as claimed inclaim 41 wherein the second fraction of fuel is injected about 20 toabout 60 degrees before the compression stroke top dead center. 46.Method of operating a direct-injection, four-stroke, internal combustionengine as claimed in claim 42 wherein the second fraction of fuel isinjected about 20 to about 60 degrees before the compression stroke topdead center.
 47. Method of operating a four-stroke internal combustionengine including a variable volume combustion chamber defined by apiston reciprocating within a cylinder between top-dead center andbottom-dead center points and at least one intake valve and one exhaustvalve controlled during repetitive, sequential exhaust, intake,compression and expansion strokes of said piston comprising: providing aclosed exhaust valve and a closed intake valve during an expansionstroke of said piston; providing an open exhaust valve and an openintake valve during an exhaust stroke of said piston to expel combustedgases to an exhaust passage and an intake passage, respectively;providing a closed exhaust valve and a closed intake valve during anintake stroke of said piston to establish a low pressure conditionwithin the combustion chamber; providing an open intake valve duringsaid intake stroke of said piston to ingest combusted gases and freshair into said combustion chamber; and, providing a closed exhaust valveand a closed intake valve during a compression stroke of said piston.48. Method of operating a four-stroke internal combustion engine asclaimed in claim 47 wherein closure of the exhaust valve occurs at anabsolute phase angle relative to exhaust stroke top dead center aboutnot greater than the phase angle after exhaust stroke top dead center atwhich opening of the intake valve occurs during the intake stroke of thepiston.
 49. Method of operating a four-stroke internal combustion engineas claimed in claim 48 wherein the closure of the exhaust valve occursabout exhaust stroke top dead center to about 20 degrees after exhauststroke top dead center.
 50. Method of operating a four-stroke internalcombustion engine as claimed in claim 47 wherein the opening of theintake valve during the intake stroke of the piston occurs about 30degrees after exhaust stroke top dead center to about 90 degrees afterexhaust stroke top dead center.
 51. Method of operating a four-strokeinternal combustion engine as claimed in claim 47 wherein opening of theintake valve that is open during said exhaust stroke of said pistonoccurs about 20 to about 90 degrees after the exhaust valve opening. 52.Method of operating a four-stroke, internal combustion engine as claimedin claim 47 further comprising: providing to the combustion chamberduring the intake stroke a first fraction of fuel of a total combustioncycle fuel requirement; and providing to the combustion chamber duringthe compression stroke a second fraction of fuel of about the differencebetween the total combustion cycle fuel requirement and said firstfraction of fuel.
 53. Method of operating a four-stroke, internalcombustion engine as claimed in claim 52 wherein the first fraction offuel comprises about 10 to about 50 percent of the total combustioncycle fuel requirement.
 54. Method of operating a direct-injection,four-stroke, internal combustion engine as claimed in claim 52 whereinsaid first fraction of fuel is injected about 0 to about 90 degreesafter exhaust stroke top dead center.
 55. Method of operating adirect-injection, four-stroke, internal combustion engine as claimed inclaim 52 wherein the first fraction of fuel comprises about 10 to about50 percent of the total combustion cycle fuel requirement and furtherwherein said first fraction of fuel is injected about 0 to about 90degrees after exhaust stroke top dead center.
 56. Method of operating adirect-injection, four-stroke, internal combustion engine as claimed inclaim 52 wherein the second fraction of fuel is injected about 20 toabout 60 degrees before the compression stroke top dead center. 57.Method of operating a direct-injection, four-stroke, internal combustionengine as claimed in claim 53 wherein the second fraction of fuel isinjected about 20 to about 60 degrees before the compression stroke topdead center.
 58. Method of operating a direct-injection, four-stroke,internal combustion engine as claimed in claim 54 wherein the secondfraction of fuel is injected about 20 to about 60 degrees before thecompression stroke top dead center.
 59. Method of operating adirect-injection, four-stroke, internal combustion engine as claimed inclaim 55 wherein the second fraction of fuel is injected about 20 toabout 60 degrees before the compression stroke top dead center.